In the field of video displays, the practice of "overscanning" to insure that the picture fills the screen is well known. To illustrate the practice of overscanning, the practice's application in rear projection color television is considered. In a typical rear projection color television, as in many types of video displays, pictures are formed on a screen by combining multiple beams of light. Usually, a set includes three cathode ray tubes (CRTs), one for each of the primary colors--red, blue and green. The set combines the three monochromatic beams emanating from the CRTs to produce full color television pictures.
FIG. 1 is a plan view of a typical rear projection television set. The components of the set are housed within a cabinet 10, and they include: a CRT 12, a lens 14, a mirror 16, and a screen 18. In actuality, the model set includes three CRTs and multiple lenses for each CRT, although for clarity, only the single CRT and the single lens are shown in the figure. The light from the CRT passes through the lens and illuminates the mirror which, in turn, reflects the light onto the screen for observation by the viewer. The screen is secured within the cabinet with the aid of a frame. FIG. 2 is an isometric view which shows the position of a frame 36 relative to the cabinet and screen.
FIG. 3 illustrates the relationship between the three CRTs of the typical set. As can be seen from the figure, CRTs 12, 20 and 22 are matched respectively with lenses 14, 24 and 26, and the CRTs are aligned so that their beams converge. The practice of overscanning concerns what happens when the CRT beams are scanning toward the edge of the screen.
FIG. 4 is a diagram that shows what happens when a CRT scan reaches the edge of the screen. Notably, the scan does not stop but continues on so that light emitted from the CRT is incident on the frame. That portion of the television picture signal that corresponds to the overscanned CRT emissions, and the overscanned CRT emissions themselves, are commonly referred to as the "blanking signal".
FIG. 5 illustrates overscanning from the viewer's perspective. The figure shows an exemplary relationship between an area scanned by the CRT 38, the screen, and the frame. When the CRT beams are properly aligned with the center of the screen, the scanned area shown extends beyond the screen dimensions by a distance "d" in all directions, as shown. Thus, even if the beams were to become misaligned by an amount .ltoreq.d, the viewer would still see a full display screen.